[LLVMdev] Turning sub-target features on/off (e.g. Altivec on PowerPC)

[Xerxes Rånby]

On 2010-03-18 15:38, Sergei Dyshel wrote:
> Hello,
>
> I'm using Mono with experimental LLVM backend support on PowerPC. I
> noticed that although LLVM's IR contains SIMD instructions the
> assembly produced doesn't contain any Altivec instructions and my
> PowerPC970 machine of course has Altivec support. Isn't there some
> kind of autodetection? I searched in Target sources but only found out
> that Altivec is disabled by default.
No there are no automatic autodetection for ppc.
AFAIK only the X86 backend currently have some autodetection code buried
in the target backends. The habit to have autodetection code in the
target backend are discouraged and X86 are currently moving code out
from the backend into two new API found in LLVM 2.7 that can be used by
clients to find out of the cpu capabilities.
lib/System/Hosts.cpp
This are the right place to add new PPC cpu/attributes autodetection code.
http://llvm.org/bugs/show_bug.cgi?id=5389

>
> Can I turn Altivec support on (and off, I need both cases) from the
> code? Some info: Mono's utilizes LLVM in this way:
>
> LLVMInitializePowerPCTarget ();
> LLVMInitializePowerPCTargetInfo ();
> ExecutionEngine *EE = ExecutionEngine::createJIT ( ... );
> fpm = new FunctionPassManager ( ... );
> fpm->add(new TargetData(*EE->getTargetData()));

I you use the EngineBuilder instead of calling createJIT directly then
you can set the cpu type and select cpu attributes using mattr easily,
this are how lli does it:

InitializeNativeTarget();   // use this instead of
LLVMInitializePowerPCTarget (); and LLVMInitializePowerPCTargetInfo ();

EngineBuilder builder(Mod);
builder.setMArch(MArch); // set arch  ppc_32 ppc_64 etc.
builder.setMCPU(MCPU); // set desired cpu type for code generation
builder.setMAttrs(MAttrs); // set desired cpu attributes (enable Altivec
etc)
EE = builder.create();
...
fpm = new FunctionPassManager ( ... );
...

>
> and then uses ExecutionEngine object for compiling functions.
> -- 
> Regards,
> Sergei Dyshel
>
>
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>   
Cheers
Xerxes
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